Geolocation refers to techniques for determining the geographic location of an object. Various types of geolocation exist. The present invention is applicable to environments where the object to be geolocated emits a signal. In such environments, various measurements can be performed on the received signal to estimate the location of the emitting object. For example, a receiver can perform angle (or direction) of arrival techniques to estimate the angle between the emitting object and the boresight vector of the antenna's receiver.
Angle of arrival techniques are often performed by detecting phase differences at a number of antennas that receive the signal emitted by the object. In such systems, each antenna is coupled to the system via various analog RF components (e.g., LNAs, up/down converters, cables, etc.) whose characteristics vary with temperature and frequency (i.e., they will cause an unknown phase rotation between the antenna and the angle of arrival system). The characteristics will also vary between components of the same type. For example, multiple RF cables, even if they have the same length, will rarely cause the same phase shift to a signal. Due to these variations and inconsistencies, it can be difficult to estimate the angle of arrival with high precision.
Generally, there are two options for addressing the phase rotation caused by these RF components. First, specialized RF components that minimize the effects of temperature and frequency on the system can be employed. However, such components are expensive and still do not fully compensate for temperature- and frequency-based variations. Second, specialized measuring equipment can be employed to measure the phase rotation caused by the RF components. However, this type of calibration would require disconnecting the antenna array from the angle of arrival system and is therefore unfeasible in many scenarios.
Antenna boresighting refers to determining the direction (or vector) in which the antenna is pointing. In the case of a single antenna, the boresight vector is the direction of maximum gain relative to a reference frame. Therefore to identify the boresight vector of a single antenna, the antenna can be steered until the gain of the received signal is maximized. The orientation of the antenna at this point would define the reference plane for angle of arrival purposes. However, with an antenna array, each antenna's boresight vector will typically be pointed in a slightly different direction thereby making this boresighting technique unsuitable, or at least inaccurate, for antenna arrays. In particular, it is typically necessary to use the plane in which the elements of the antenna array are situated as the reference plane for angle of arrival purposes. However, there is no guarantee that the boresight vector of a particular element will be orthogonal to this plane.
FIG. 1 provides a simple diagram to illustrate this issue. In this figure, an antenna array 111 is assumed to have three antennas 111a-111c arranged along a plane. The dashed lines represent a line perpendicular to this plane. As shown, the maximum gain of each antenna is offset slightly at −2°, 3°, and 6° respectively from this perpendicular line. Therefore, if the boresight vector of any of antennas 111a-111c is chosen as the boresight vector of antenna array 111 (hereinafter “the virtual boresight vector”), an error would be caused in any subsequently performed angle of arrival calculations due to the fact that antenna array 111 will not be pointing in the direction that it is assumed to be pointing.